There has been explosive growth in the area of wireless communications. A few years ago, the sight of a person speaking into a cellular phone was a curiosity; now it is commonplace. Communication via cellular phones is supported by wireless telecommunications systems. Such systems service a particular geographic area that is partitioned into a number of spatially-distinct areas called "cells." Each cell usually has an irregular shape (though idealized as a hexagon) that depends on terrain topography. Typically, each cell contains a base station, which includes, among other equipment, radios and antennas that the base station uses to communicate with the wireless terminals (e.g., cellular phones) in that cell.
The antenna used for transmitting signals from a base station is typically a linear phased-array antenna. A phased-array antenna is a directive antenna having several individual, suitably-spaced radiating antennas, or elements. The response of each radiating element is a function of the specific phase and amplitude of a signal applied to the element. The phased array generates a radiation pattern ("beam") characterized by a main lobe and side lobes that is determined by the collective action of all the radiating elements in the array.
It may be desirable, at times, to adjust the geographic coverage of a particular base station. This can be accomplished changing the azimuth ("beam steering") or elevation ("beam tilting") or both (henceforth "beam steering"), of the beam generated by a base station's transmit antenna.
The beam generated by a linear phased-array antenna can be tilted by mechanically rotating the entire antenna array, or by employing a progressive element-to-element phase shift. The two different approaches are not equivalent in terms of their effect on an antenna's radiation pattern. Down tilting via progressive phase shift results in a decrease in peak gain efficiency while the azimuth radiation pattern stays the same. On the other hand, mechanical down tilting can significantly distort the azimuth radiation pattern when projected on the ground-level coverage zone within a cell. Moreover, while progressive phase shifting provides the ability to beam "shape," mechanical down tilting does not. For the foregoing reasons, it is generally preferable to use phase shifting rather than mechanical down tilting.
Progressive phase shift can be accomplished using phase shifters. Most conventional phase shifters suffer from various drawbacks that makes implementation into phase arrays problematic. For example, some conventional phase shifters, such as switchable delay lines and ferrites, are large (and expensive). Integrating such large-sized phase shifters into phased-array antennas often requires modification of the feed network. Other conventional phase shifters, such as solid-state hybrid-coupled-diode phase shifters and thin-film ferrites, disadvantageously exhibit substantial nonlinearity. Moreover, solid-state hybrid-coupled-diode phase shifters have high insertion loss requiring that amplifiers be used at the top of a base station tower to increase signal levels. At the high power levels required for transmission, such amplifiers are heavy, big and expensive. While at the lower power levels characterizing "receive" operation, such amplifiers are considerably smaller and less expensive, it is still generally undesirable to have such active RF electronics at the top of a tower. Still other conventional phase shifters, such as "sliding contact shifters," suffer from corrosion and electrical contact problems over time. In one implementation of a sliding-contact phase shifter, coaxial lines "telescope" into or out of one another such that the line length of the phase shifter, and hence the phase imparted thereby, is changed.
Thus, there is a need for a steerable phased-array antenna having a phase-shifter array that avoids the drawbacks of the prior art and is readily implemented into antenna feed networks without substantial modifications thereto due to the size, weight, etc., of the phase-shifting array.